Methods to enhance matrix acidizing in low permeabilty reservoirs

ABSTRACT

The subject disclosure relates to matrix acidizing. More specifically, the subject disclosure relates to manipulating downhole pressure to promote in-situ mixing. In particular, downhole pressure is temporarily reduced to allow churning of the dissolved CO 2  to facilitate mixing efficiency within the wormholes and the matrix around the wormholes.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/691,512 filed Aug. 21, 2012, which is incorporated herein by reference in its entirety.

FIELD

The subject disclosure generally relates to matrix acidizing. More particularly, the subject disclosure relates to methods of enhancing matrix acidizing in low permeability reservoirs.

BACKGROUND

Matrix acidizing is a widely practiced treatment of oil/gas wells in carbonate reservoirs. Matrix acidizing operations involve injecting acid into an isolated treatment zone at pressures below the fracture pressure of the formation. The injected acid dissolves the formation rock to form channels or wormholes, which extends the wellbore drainage radius. The purpose of this stimulation technique is to increase the production rate by increasing the near borehole equivalent permeability. The acidizing treatment could be enhanced by increasing the depth of penetration into the formation of the wormholes.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

According to some embodiments, a method is described for acid treating a subterranean reservoir formation from a wellbore penetrating the formation. The method includes: isolating a treatment zone of the formation; using pumping equipment, pumping an acidic fluid into the treatment zone of the formation so as to form a plurality of conductive channels extending from the wellbore into the formation; and controlling the pumping equipment so as to intentionally repeatedly decrease and increase pressure of the pumped acidic fluid in order to extend depths of the conductive channels into the formation. According to some embodiments, the conductive channels are wormholes, and the decreasing and increasing pressure extends the depths of the channels by enhancing in-situ fluid mixing near distal tips of the channels. The pressure is also controlled so as to not cause fracturing of the formation. The pumping equipment can be controlled in various ways to decrease and increase the fluid pressure including: varying pumping speed; repeatedly ceasing pumping; and repeatedly drawing down pressure in the formation. According to some embodiments, the formation is a low-permeability carbonate formation and the acid used is hydrochloric acid.

According to some embodiments, a system is described for acid treating a subterranean reservoir formation surrounding a wellbore. The system includes: tubing configured to run from a surface wellsite through the wellbore to an isolated treatment zone in the subterranean formation; pumping equipment in fluid communication with the tubing and configured to pump an acidic fluid through the tubing and into the isolated treatment zone thereby forming a plurality of wormholes extending from the wellbore into the formation; and a processing system configured to control the pumping equipment in order to decrease and increase pressure of the pumped acidic fluid to extend depths of the wormholes into the formation while maintaining the pressure at levels so as to avoid fracturing the formation. According to some embodiments, the tubing is coiled tubing, and the system further comprises one or more packers to isolate the treatment zone.

Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

FIG. 1 illustrates a system for matrix acidizing a low-permeability reservoir, according to some embodiments;

FIG. 2 is a perspective view of an example wormhole formed in a core sample, according to some embodiments;

FIG. 3 is a graph depicting the differential pressure profiles along the core sections as a wormhole propagates; and

FIG. 4 is a flow chart illustrating aspects of a method for matrix acidizing a low-permeability reservoir, according to some embodiments.

DETAILED DESCRIPTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.

It has been found that the depth a wormhole can penetrate into the formation depends on the conditions at the wormhole tip where the reaction rate generally decreases continuously as the acid concentration becomes lower due to acid reaction. One of the reaction products, carbon dioxide, further changes the reaction rate. It has been found that in low-permeability formations, the reactive acidic fluids at wormhole tips may become stagnant due to lack of fluid loss at the wormhole tip. At reservoir conditions, dissolved CO₂ may negatively impact the acidizing efficiency by preventing weak acid dissociation to completion.

FIG. 1 illustrates a system for matrix acidizing a low-permeability reservoir, according to some embodiments. A treatment zone 102, which according to some embodiments is a low-permeability carbonate reservoir rock formation, is being treated with an acid. An injection tubing 116 is deployed via coiled tubing truck 120 into wellbore 114 that extends from the well head 112 on the surface to the low-permeability zone 102. The treatment zone in formation 102 is isolated via one or more packers, such as packer 132. Equipment at the wellsite 110 includes one or more other service vehicles, such as 122, as well as mixing and pumping equipment 124. In the example shown wellbore 114 has an uncased section 118 in the vicinity of treatment zone 102. Also shown in FIG. 1 is data processing unit 150, which according to some embodiments includes a central processing system 144, a storage system 142, communications and input/output modules 140, a user display 146 and a user input system 148. The data processing unit 150 may be located on one or the other of trucks 120 and 122 and/or may be located in other facilities at wellsite 110 or in some remote location. The processing unit 150 is used to monitor pressure and control pumping equipment that may be located in one or more of trucks 120, 122 and mixing and pumping equipment 124.

The acid injection into zone 102 causes a number of wormholes to form in the formation as is shown by the solid lines (such as solid line 134) leading from the wellbore into the formation. According to some embodiments, downhole pressure is manipulated to promote in-situ mixing. More specifically, according to some embodiments, downhole pressure is manipulated to promote in-situ mixing by temporarily reducing the downhole pressure to allow churning of the dissolved CO₂ to facilitate the mixing efficiency within the wormholes and matrix around the wormholes. Enhancing the in-situ mixing of fluids increases the local acid mass transfer and reduce dissolved CO₂ locally, especially at the wormhole tips. The induced in-situ mixing causes the wormholes to lengthen, that is penetrate to greater depths into the formation 102 as is depicted by the dotted lines such as dotted line 136.

The acidizing system shown in FIG. 1 is thus configured to enhance in-situ mixing of fluids during acid injection in tight carbonate reservoirs such a treatment zone 102. Pulsed or intermittent pumping and drawdown are used instead of pumping continuously, which is common in conventional acidizing operations. The pressure drawdown in wormholes facilitates the flow back of fluids and causes expansion of dissolved CO₂ in the solution, resulting in better in-situ mixing. Increasing the injection rate in a pumping cycle may deliver fresh acid to the wormhole tips, thus allowing effective reactions and channel penetration at the tip of the wormhole.

Numerical stimulation of wormhole development was used which confirms that acid is indeed depleted at the wormhole tip due to acid reaction. FIG. 2 is a perspective view of an example wormhole formed in a core sample, according to some embodiments. Shown is a wormhole 210 in a low permeability limestone 12-inch core sample 200. The wormhole 210 was formed using an acid injection experiment. It can be seen that fresh acid near the entrance of acid injection at the proximal end 212 (corresponding to the base of the wormhole at the borehole wall) of wormhole 210 continued to dissolve the carbonate rock resulting in a large cavity whereas the distal tip 214 (corresponding to the end furthest from the borehole) of the wormhole ceased to propagate. In low-permeability reservoirs, when pumping capacity reaches the limit or pumping rate reaches the designed optimum, using pulsed or intermittent pumping enhances the fluid mixing in the wormhole and brings fresh acid forward. Furthermore, it is believed that pressure drawdown makes the dissolved CO₂ expand to further enhance the mixing.

FIG. 3 is a graph depicting the differential pressure profiles along the core sections as a wormhole propagates. The core sample for the experiment of FIG. 3 has a permeability of about 0.6 mD. 15% HCL was injected into the core at 2 ml/min and the wormhole rapidly penetrated the rock initially. As can be seen by the pressure profiles 310, 312, 314, 316 and 318, the penetration gradually slowed down and eventually stopped as the acid strength at the wormhole tip could not support further wormhole propagation. After 2.7 pore volumes of acid was injected without achieving acid breakthrough of the full 12 inch core, the injection rate was increased from 2 ml/min to 6 ml/min. The higher rate delivered fresh acid to the tip of the wormhole and therefore allowed the wormhole to continue through the last section of the core as can be seem by profile 318. This shows that when injecting at 2 ml/min, the acid was almost depleted at the tip of the wormhole when it reached 9.6 inches. In the field, however, injection rates are limited by factors such as pumping capacity and fracturing pressure. Furthermore, higher than optimum pumping rates may lead to highly ramified wormholes that do not penetrate far into the formation. Thus, according to some embodiments, repeatedly reducing the pressure provides an effective means of extending the depth of wormholes.

FIG. 4 is a flow chart illustrating aspects of a method for matrix acidizing a low-permeability reservoir, according to some embodiments. In block 410, the formation zone of low-permeability rock that is selected for treatment is isolated. In block 412, acid, such as HCl is injected into the isolated zone at pressures below the fracturing pressure. The acid causes the formation of multiple wormholes extending from the borehole wall into the formation rock. In block 414, the pumping equipment is controlled, such as using processing unit 150 shown in FIG. 1, so as to repeatedly decrease (block 416) and thereafter increase (block 418) the fluid pressure in the isolated region. The pressure fluctuations enhance in-situ fluid mixing at the distal tips of the wormholes thereby causing an increase in the length (and penetration depth) of the wormholes. The deeper wormholes act to improve the production or injection flow capacity of the wellbore.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. 

What is claimed is:
 1. A method for acid treating a subterranean reservoir formation from a wellbore penetrating the formation, the method comprising: isolating a treatment zone of the formation; using pumping equipment, pumping an acidic fluid into the treatment zone of the formation so as to form a plurality of conductive channels extending from the wellbore into the formation; and controlling the pumping equipment so as to intentionally repeatedly decrease and increase pressure of the pumped acidic fluid so as to extend depths of the conductive channels into the formation.
 2. A method according to claim 1, wherein the conductive channels are wormholes.
 3. A method according to claim 2, wherein the decreasing and increasing pressure extends the depths of the conductive channels at least in part by enhancing in-situ fluid mixing near distal tips of the conductive channels.
 4. A method according to claim 1, wherein fluid pressure is controlled so as not to cause fracturing of the formation.
 5. A method according to claim 1, wherein controlling the pumping equipment includes varying a pumping speed.
 6. A method according to claim 1, wherein controlling the pumping equipment includes repeatedly ceasing pumping.
 7. A method according to claim 1, wherein controlling the pumping equipment includes repeatedly drawing down pressure in the formation.
 8. A method according to claim 1, wherein the formation is a low-permeability formation.
 9. A method according to claim 1, wherein the formation is a carbonate formation.
 10. A method according to claim 1, wherein the acidic fluid contains hydrochloric acid.
 11. A system for acid treating a subterranean reservoir formation surrounding a wellbore, the system comprising: tubing configured to run from a surface wellsite through the wellbore to an isolated treatment zone in the subterranean formation; pumping equipment in fluid communication with the tubing and configured to pump an acidic fluid through the tubing and into the isolated treatment zone thereby forming a plurality of wormholes extending from the wellbore into the formation; and a processing system configured to control the pumping equipment in order to decrease and increase pressure of the pumped acidic fluid to extend depths of the wormholes into the formation while maintaining the pressure at levels so as to avoid fracturing the formation.
 12. A system according to claim 11, wherein the decreasing and increasing pressure extends the depths of the wormholes at least in part by enhancing in-situ fluid mixing near distal tips of the wormholes.
 13. A system according to claim 11, wherein the control of the pumping system includes repeatedly ceasing pumping.
 14. A system according to claim 11, wherein the control of the pumping system includes repeatedly drawing down pressure in the formation.
 15. A system according to claim 11, wherein the control of the pumping system includes varying a pumping speed.
 16. A system according to claim 11, wherein the formation is a low-permeability carbonate formation.
 17. A system according to claim 11, wherein the tubing is coiled tubing, and the system further comprises one or more packers to isolate the treatment zone. 